Monday, September 22, 2014

I've written a couple of posts this year and in the past about the southwestern monsoon season in the U.S. Even with maps, charts, and photos it's difficult for those who have never experienced that part of the country to imagine the scope of the weather that occurs during this important season. Today I came across a video that condenses the 2014 monsoon into eight minutes of boiling cumulus clouds, intense rain shafts, hail, haboobs, lightning, and desert scenery. Photographer Mike Olbinski spent the summer driving 14,000 miles to capture the images that make up this time lapse video.

Maps and charts really only tell the technical part of the story. A video such as this one easily explains why so many are fascinated with the weather.

Be sure to watch this full screen and turn up your speakers. You will want to view this more than once to catch all of the details.

By the way, if you do want a "technical" explanation of the southwestern monsoon see a previous post here.

Wednesday, September 17, 2014

A few days ago Odile was a Category Four Hurricane off the west coast of Mexico, and as of this morning was still a weak tropical storm. Odile fell apart over northwestern Mexico during the day, and is now just a shell of its former self. However, the remnants of Odile will still have quite an impact on weather for the next few days, first across the southwestern U.S. and then perhaps across the extreme southern U.S.

Map shows the expected position of the remnants of Odile [D] at 11:00 p.m. PDT September 17.

The remnants of Odile were already feeding plenty of moisture into the Desert Southwest. Storms were bubbling up today across Arizona and New Mexico while some isolated storms popped up in southern California.

Yesterday there were some rare severe thunderstorms in southern California. Wind damage was reported in and around San Diego, where numerous trees and light poles were toppled and small planes were flipped over at Montgomery Field Airport. One inch hail was reported near Joshua Tree, CA in San Bernadino County.

San Diego radar image for 3:15 p.m. PDT September 16. Storms were moving to the west.

Conditions across southern Arizona and New Mexico today were decidedly tropical. It was quite humid with dewpoints generally in the mid to upper 60s. The dewpoint in Phoenix this afternoon reached 72°F, a level that is rarely seen there. Swamp coolers (evaporative air conditioners) don't work very well when the dewpoint is high.

There are a combination of factors that will make the next few days messy, to say the least, in the Desert Southwest. CoCoRaHS observers in the region will be getting a lot of measurements in over the next several days. Although we are the tail end of the summer monsoon season, there will be heavy showers and thunderstorms because of the plentiful moisture being fed into the region. Weak flow in the upper atmosphere will provide for little steering of the showers and thunderstorms, and those that develop could remain over a relatively small area for much of the storm's lifetime. That means potentially heavy rain for a long period of time. That same weak upper flow also means that the leftovers from Odile will be slow to move out of the region.

500 millibar map for 5:00 a.m. PDT this morning. The circulation of Odile is caught in a ridge over the Southwest.

The NWS Weather Prediction Center is expecting as much as 4 inches of rain from southeastern Arizona through southern Arizona and into western Texas, with some locally higher amounts possible.

Rainfall forecast through 5:00 p.m. PDT Saturday, September 20.

Flood watches are in effect for large portions of Arizona and New Mexico.

The showers and thunderstorms are most numerous tonight south of the Arizona-Mexico border but are steadily feeding northward. The Tucson NWS office has a nice summary of daily and monthly rainfall records for a number of locations in southern Arizona on their web site.

Monday, September 15, 2014

Unless you weren't paying attention at all to the weather last week you have heard about the snow in the Rockies and northern Plains as well as the unseasonably cool weather that followed for much of the eastern three-quarters of the country. So how bad was it?

The snow forecasts were pretty much on target, though amounts in a few places (notably mountain areas) were a little higher than expected. The CoCoRaHS observer at WY-SH-33, Sheridan 15.7 W on the east slopes of the Bighorn range measured a storm total of 19.0 inches of snow. Snow fell along the Front Range in Colorado as far south as Denver, with a half inch of accumulation in Boulder northwest of Denver. The accumulating snow reached as far east as Rapid City, SD with 3.5 inches, the earliest measurable snow on record. The old record for Rapid City was 2.8 inches on September 15, 1903. Six to 8 inches of snow fell in the Black Hills during this storm. North Platte, NE picked up a trace of snow, the earliest on record there.

72-hour snowfall accumulation for the period ending 7:00 a.m. local on September 12

Obviously that snow would not have been possible without some pretty cold air for this time of year. The cold air spilled south behind this storm system. On Friday, September 12 Cloud Peak, WY in the Bighorn Mountains recorded the nation's lowest temperature of 4°F. By Saturday morning, September 13 the cold air had pushed as far south as central Texas and New Mexico.

Minimum temperatures for the period from 7:00 p.m. CDT September 12 to 7:00 a.m. September 13

Temperatures for September 12-14 were as much as 20°F below normal in the Central Plains, with much of the middle third of the country from 10°F to 15°F below normal.

In the northern tier of states the first freeze of the season was recorded from Wyoming east into northern Kansas and northwestern Wisconsin and northern lower Michigan.

Map of locations recording first 32°F freeze

Fortunately, a hard freeze (28°F) was limited to Wyoming, Colorado. the far western Dakotas, western Nebraska, and scattered areas in northern Minnesota.

Map of locations recording first 28°F freeze

For most areas the first occurrence of 32°F was about two to four weeks early.

Median date of first occurrence of 32°F in the fall.

The weather will gradually warm this week and it looks like we'll enjoy some "normal" September weather for the next week to ten days

Wednesday, September 10, 2014

The forecast for much of the central and eastern U.S. has been calling for much cooler weather over the next few days. The cold air that will spread over the U.S. the rest of this week has already plunged into southern Canada and the northern U.S. Rockies. Calgary, Alberta has been hit hard by heavy, wet snow this week. On Monday Calgary picked up 11.8 cm (4.5 inches) of new snow, and on Tuesday an additional 1.3 cm (0.5 inch), and had 10 cm (4 inches) on the ground. This was all following a weekend with temperatures in the mid 70s Fahrenheit. The heavy, wet snow brought down trees that still were leafed out, and those in turn took down power lines. This morning an estimated 30,000 customers were without power. In addition to downed trees and power lines there were numerous traffic accidents, sort of typical for the first snow of the season, even in Canada. Some flights were delayed or cancelled at Calgary International Airport. Interestingly, schools remained open except for a few that had power outages.

This probably was a common sentiment
in Calgary. Photo by Deanna Allen

September snow is not that unusual for Calgary, with about 2 days of measurable snow on average. The snow Calgary has received this week is what they would normally receive for September and October combined. The snow continued today, and this morning Environment Canada issued a Snowfall Warning for the City of Calgary today with another 10 to 15 cm (4 to 6 inches) of snow expected. Except for damaged trees the snow will soon be a memory as a warmup is expected through the weekend and temperatures will be in the 70s Fahrenheit by early next week.

The cold air doesn't stop at the border, of course. Snow was falling in higher elevations in northern Montana today and Winter storm Warnings and Winter Weather Advisories are in effect until midday Thursday for areas on the east side of the Rockies as far south as northeastern Wyoming. Some snow also dusted the higher elevations in northern Colorado today.

View from Montana DOT camera at Two Medicine River Bridge (elevation 4900 ft) near East Glacier at 6:19 p.m. MDT September 10

The higher mountains and passes could receive 6 to 12 inches of snow while the north-central plains pick up 1 to 3 inches. The southwestern valleys of Montana and lower elevations in Wyoming will see mostly a rain/snow mix.

Watches, warnings, and advisories in effect as of 6:27 p.m. MDT

As this cold air continues to spill south and east it will usher in an extended period of cooler than normal weather for the eastern two thirds of the country.

Minimum temperature forecast for Saturday, September 13

Maximum temperature forecast and departure from normal for Wednesday, September 17.

Tuesday, September 9, 2014

It's not often the rainfall in the Desert Southwest will make the news twice in less than a month's time, but this is one of those times. Late Sunday night into early Monday morning, September 8 thunderstorms feeding on a plume of moisture associated with what remained of Hurricane Norbert flushed out over the Phoenix metropolitan area. Record rainfall amounts occurred in a span of only 8 hours or so in an area not accustomed to multi-inch rainfalls.

Two clusters of storms merged. One moved from southwest to northeast west of Phoenix, and the second developed southeast of Phoenix and moved northwest, where it merged with the first cluster.

Reflectivity image for 11:47 p.m. MST September 7 (left) and 2:03 a.m. MST September 8 showing the two clusters of storms approaching the Phoenix area.

Water vapor satellite image for 3:45 a.m. MST. The complex of storms over southern Arizona can be readily seen.

Radar reflectivity image at 3:20 a.m. MST September 8.

The rain in the Phoenix area continued through mid-morning before tapering off. Rainfall amounts were unusually heavy and widespread.

Phoenix Sky Harbor Airport set new records for the highest daily rainfall for September 8 and the highest rainfall in a calendar day. The previous records had been in place 75 years or more.

Then, from late morning through early afternoon Monday heavy rain fell in the Tucson area. Tuscon received 1.87 inches of rain, a new record for September 8. The old record was 0.94 inch in 1919. More information on the Phoenix rain can be found on this NWS Phoenix web page.

CoCoRaHS map for the Tucson area for the 24-hour period ending the morning of September 9.

About the same time the rain was tapering off in the Phoenix area thunderstorms erupted over southern Nevada northeast of Las Vegas in northeast Clark County. More than 4 inches of rain was recorded in some locations. Flooding damaged some homes in the small town of Moapa 50 miles northeast Las Vegas just off Interstate 15, forced the evacuation of an Indian reservation and sent torrents of water across Interstate 15. Much of this rain fell in only a two-hour period, pushing the Virgin River to near-flood stage.

Here is a video showing some of the flooding on Interstate 15 on Monday.

The heavy rain story isn't over yet. Heavy rain is falling tonight along a stationary front draped across the Midwest, More than 3 inches of rain has already been reported in Iowa and northern Missouri, and flash flood warnings and watches are in effect for a large portion of the Midwest.

Watches, warnings, and advisories in effect as of 11:45 p.m. CDT September 8.

Thursday, September 4, 2014

On early Thursday morning a somewhat rare and unusual phenomenon occurred in the High Plains. A series of heat bursts were observed in western Nebraska and central and southern South Dakota.

A heat burst is a downdraft of very warm and dry air, associated with decaying showers or thunderstorms that usually occurs in the evening or overnight. Heat bursts at the surface are characterized by dramatic rises is temperature, decreases in dew point, and strong gusty winds.

A heat burst develops when rain falls through a very dry layer in the middle levels of the atmosphere. As the rain falls through the layer of very dry air it evaporates. As the water evaporates, the air cools and accelerates toward the surface. Once all of the precipitation has evaporated the air can no longer be cooled by the evaporation process. This dense air continues to accelerate toward the surface but now rapidly warms as it compresses at a rate of 5.5°F per 1000 feet (called the dry adiabatic lapse rate). This downdraft hits the ground and spreads out in a similar fashion to a downburst from a severe thunderstorm.

This is the radar image from Aberdeen, SD shows the decaying showers and thunderstorms over southern South Dakota generally south of Huron (HON).

Temperature, humidity, and wind plots from White Lake, SD showing the heat burst. The temperature at White Lake rose from 73°F to 89°F and the humidity dropped from 92% to 24% all in the span of 25 minutes,
and the peak wind gust was 47 mph..

I experienced the effects of a heat burst in July 1995 while in Scottsdale, AZ attending a meeting. It was in July and the high temperature for the day was 121°F. That night a number of us where sitting around the hotel pool around 11:00 p.m. when the winds picked up and the temperature went from about 102°F to 111°F in a matter of minutes. That heat burst came from a decaying thunderstorm over Phoenix. The effects were a little more dramatic closer to Phoenix and there was some wind damage as well. It was a very odd thing to experience, especially at those temperatures.